Niche-structured assemblages of exotic earthworms in headwater streambanks in eastern New York State, USA

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In eastern North America, most Lumbricid earthworms found today above the post-Wisconsin Glacial boundary are non-indigenous, some with marked potential as invasive and ecologically disruptive species. These earthworms tend to appear as assemblages, not solitary species. Among potential explanations are successional facilitation among taxa, anthropogenic dispersal mechanisms that promote multi-species introductions, or simply shared habitat determinants that lead to accumulations through various dispersal pathways. We examined headwater streambanks in higher elevation forests of eastern New York State, and found up to 15 lumbricid earthworm species per 50 m-long reach among 14 sites divided between two montane regions, the northern Catskill Mountains and the Helderberg Plateau of eastern New York State.After pooling six surveys per site taken over two years, we found consistent differences between the two regions. Although most species (14/18) were found in both regions, their relative abundances and assortment patterns differed. Eisenoides lonnbergi and Bimastos rubidus, two native species were more prevalent in the Catskills, and represented new records for both regions. Ordination (CCA) on a set of measured environmental parameters revealed that most species were distributed along a soil pH-tree composition gradient. At the site level, clustering was strongly regional, despite a relatively small distance (<25 km) separating the regions. In all, several determinants may be at work in shaping observed earthworm assemblages. However, we did not detect evidence for interspecific interactions as a structuring element. When tested with a null model simulation, observed pairwise species associations did not depart significantly from random expectations (z-tests, p > 0.30 for both regions). Furthermore, the riparian zones we surveyed are relatively dynamic settings, so the stability of these novel assemblages is uncertain. With other invasive earthworm species now spreading throughout New York State, that uncertainty may grow.